The table below details the combinations of bets available for various numbers of selections

Bets & Selections

Sels

Dbs

Trb

4's

5's

6's

7's

8's

9's

10's

Total

2

1

1

3

3

1

4

4

6

4

1

11

5

10

10

5

1

26

6

15

20

15

6

1

57

7

21

35

35

21

7

1

120

8

28

56

70

56

28

8

1

247

9

36

84

126

126

84

36

9

1

502

10

45

120

210

252

210

120

45

10

1

1013

Should you want to extend the table to include more selections, it is quite easy.
Take a close look at the table and you should see a pattern emerging....

For the line with 10 selections you can see that:

the doubles is made up of the doubles for 9 selections (36) plus the
9 selections (9) = 45

the trebles is made up of the number of trebles in 9 selections (84) plus the
number of doubles in 9 selections (36) = 120

the fourfolds is made up of the number of fourfolds in 9 selections (120) plus the
number of trebles in 9 selections (84) = 210

and so on....

Each row figure is made up of the row figure above it, added to the row figure to the
left of that figure! The explanation is that say for trebles, the same trebles are still
applicable, but what was each double, now has the new selection added to it
to make an additional treble.